Electrostatic spray gun

ABSTRACT

This invention relates to an electrostatic spray gun for the atomization and spraying of coating products. The gun includes an electro-aerodynamic generator having a conduit for the flow of a gaseous fluid at least partially at supersonic speed from an upstream convergent-divergent region containing ionization means to a downstream zone at high potential adjacent the atomization nozzle. According to the invention, means for shutting of the flow of gaseous fluid are arranged downstream of the electroaerodynamic generator, for example on the nozzle body. The electro-aerodynamic generator is preferably constructed as a removable cartridge, and means are also provided for adjusting the angle and velocity of the output spray from the gun.

United States Patent Tholome 1 June 20, 1972 s41 ELECTROSTATIC SPRAY GUN 3,246,844 4/1966 Lehman et a1 ..239/15 [72] Inventor: Roger ohm, Comm France 3,448,925 6/1969 Cross ..239ll5 [73] Assignee: Tunzini-Sames, Grenoble, France Primary ExaminerL1oyd L King Filed: In! 1970 AltorneyHolcombe-, Wethenll & Bnsebois App]. No.2 54,543

[57] ABSTRACT This invention relates to an electrostatic spray gun for the atomization and spraying of coating products. The gun includes an electro-aerodynamic generator having a conduit for the flow of a gaseous fluid at least partially at supersonic speed from an upstream convergent-divergent region containing ionization means to a downstream zone at high potential adjacent the atomization nozzle. According to the invention, means for shutting of the flow of gaseous fluid are arranged downstream of the electro-aerodynamic generator, for example on the nozzle body. The electro-aerodynamic generator is preferably constructed as a removable cartridge, and means are also provided for adjusting the angle and velocity of the output spray from the gun.

7 Claims, 6 Drawing figures PATENTED JUN 2 0 I872 SHEET 2 OF 2 ELECTROSTATIC SPRAY GUN The present invention relates to an electrostatic spray gun of the type incorporating, in the gun barrel, an electroaerodynamic generator comprising a conduit for a flow of a gaseous fluid at least partially at supersonic speed, from an upstream zone formed with a convergent-divergent region in which ionization means are arranged, to a downstream area at high potential adjacent the outlet from the said conduit. In such a generator, the gaseous fluid entrains the electric charges emitted at the level of the convergent-divergent region and therefore develops a certain electric power at the output, in the form of a current of the order of about microamperes at voltages of several tens of kilovolts, which is particularly suitable for the use of an electrostatic spray gun. In electrostatic spray guns using air for the atomization of paint and for the dispersion of solid particles, the air issuing from the electro-aerodynamic generator is at least partially used for the atomization or spraying of the coating product.

Such electro-aerodynamic generator guns have a certain number of drawbacks. Thus the intermittent breaks in operation not only require the closing of the air supply conduit but also the suppression of the excitation voltage applied to the ionization means. In fact, the ionization means are established so as to function correctly in a medium under relatively high air pressure. The absence of this air pressure, which results from the closing of the supply conduit for the air at the region of the hand grip causes a weakening in the dielectric strength and unless the excitation is cut off, the degeneration of the ionization phenomenon in an arc which is extremely harmful by reason of the erosion phenomena which it causes in the pipe.

Another drawback of guns of this type resides in the deteriorations which occur within the air conduit, and more particularly in the generator itself, by reason of the build-up in paint or solvent at the time of the interruptions in operation, during which cleaning of the nonle is usually effected. This is particularly harmful to the generator, for very small deposits of impurities are sufficient to considerably impair its performance.

One object of the invention is to provide an electrostatic spray gun of the abovementioned type wherein these drawbacks are substantially reduced or eliminated. Another object relates to a gun having multiple jets allowing during operation, the angle of the jet of sprayed product to be adjusted from a wide angle at low axial speed to a relatively narrow angle at high axial speed. Furthermore, another object of the invention relates to a generator in the form of easily interchangeable cartridges.

According to a feature of the invention the means for shutting-off the air supply are arranged downstream from the electro-aerodynamic generator. Therefore, at the time of an intermittent break in operation of the gun, the generator remains under pressure and the ionization circuit may be retained under voltage without any risk. Moreover, this pressure at the break is higher than the operational pressure since the charge losses upstream of the ionization means are eliminated. One even therefore achieves considerable reduction of the ionization phenomenon. Moreover, one can improve the static conditions within the generator by allowing a slight escape of air driving out the ozone which has formed. On the other hand, the closing of the air conduit downstream of the generator prevents any return of spray products towards this latter. Thus, since the simplest position of the means for shutting off the air conduit is in the nozzle itself, in which the downstream end of the generator discharges, practically any input of spray product into the air conduit is avoided.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a sectional view of one embodiment of a paint spray gun according to the invention;

FIG. 2 is a partial sectional view of the nozzle region of the same gun taken on another sectional plane;

FIG. 3 is a cross-sectional view along the line III-III of FIG. 2;

FIG. 4 is a cross-sectional view along the line lV-IV of FIG. 1;

FIG. 5 is a cross-sectional view along the line V-V of FIG. 2; and

FIG. 6 is a sectional view of a part of a modified embodiment.

Referring to the drawings, the gun comprises an atomisation nozzle 1, a rear hand grip 2 and a barrel 3 connecting the hand grip 2 to the noule l.

The nozzle 1 essentially comprises a nozzle body 4 having an air inlet chamber 5 of arcuate shape connecting at its up stream end with the output from an electro-aerodynamic generator 6 and at its downstream end, by means of an aperture 7, with a first annular chamber 8. The aperture 7 is in the form of a seat for a valve 10 connected to an insulating rod 1 1 controlled by a trigger l2 pivoting at 13. The annular chamber 8 connects either through axially inclined conduits 15 into a preparatory atomization chamber 16 located between the noz zle body 4 and a nozzle cap 17; or through a rear conduit 18 and an air valve 19 with a second conduit 23 itself leading at its downstream end into a second annular chamber 24, concentric with the first annular chamber 8, and outside the latter. This second annular chamber 24 leads into the preparatory atomization chamber 16 through conduits 25 all arranged in axial planes. The valve 19 is formed by a threaded body 20 having a bore 21 and controlled by a lever 22. In this manner, rotation of the control lever 22 acting on the threaded body 20 adjusts the quantity of air flowing in the conduits 18, 23 and discharging into the annular chamber 24, in order to obtain, as desired, either atomization from a whirling jet when the conduits 18 and 23 are not connected, or a relatively highly directive jet when a large quantity of air is diverted through the conduits l8 and 23 and discharged into the chamber 24 for supplying the directive conduits 25.

Due to the longitudinally and angularly offset position along the threaded body 20 of the outlets from the conduits 18, 23 leading to the formation of the bore 21 inclined to the axis of body 20 and due to the use of a screw threaded body, elongation of the leakage path between the conduit 18 and the conduit 23 is ensured in the closed position of the valve.

The atomization air which reaches the chamber 5 arrives from the output of the electro-aerodynamic generator 6 which comprises an enclosure 30 in which are located a tube 31 fonning a narrow conduit linking at its upstream end with a pipe defining a convergent-divergent passage 32 in the neck of which is situated one end of an ionization needle 33 carried by a support member 34 performated on both sides and which is supplied by an excitation circuit comprising a damping resistor 36, and a contact spring 37 on a support 38. The spring 37 is in contact with the end of the excitation cable 40 whose earthed sheath 4] is in contact with a condutive part 300 of the enclosure 30. The enclosure 30 forms a cartridge and is mounted to slide through a passage 42 in the rear of the hand grip body 2 into a rear cavity 43 in the noule member 5. It is also shown that the cartridge 30 incorporating the electroaerodynamic generator carries a finger 44 which slides in a slot of the hand grip body 2 so as to cause engagement and interlocking of the generator cartridge 30 by a bayonet type movement.

The air supplying the electro-aerodynamic generator is fed from a channel 50 in the hand grip 2, this channel 50 discharging in an annular space 51 and from there through a passage 52 through the enclosure 30 into a chamber 53 incorporating the resistor 36 from whence the air is then transferred to the generator itself through the perforations in the support 34. The trigger 12, articulated about the axis 13 integral with a rear pan of the hand grip body 2, is subjected to the action of a compression spring 55 which bears directly on the trigger l2 and also on a sealed plug 56 extending towards the rear of the hand grip body 2.

Besides controlling the air flow, as has been described previously and which is associated with the rod 11, the trigger 12 also effects control of the input of the paint at the location of the atomization nozzle. This atomisation nozzle comprises an axial annular conduit 57 supplying the paint from an input chamber 58 which can be closed by a valve seat 59 cooperating with a needle valve 60 which is connected to the trigger by means of an insulating rod 61. The paint input chamber 58 in the nozzle body communicates with the paint reservoir by means of an inclined conduit 62, with a connection 63 to which the end of a paint supply tube 64 is fixed. For ease of handling, this paint tube 64 is fixed to the lower part of the hand grip 2 by means of a support 65 in which it is integrally located.

The control for the air valve -11 and for the paint needle valve 60-61 are produced in the same manner and only the control for the paint needle valve is described, which is visible from FIG. 1 of the drawing. The compression spring 55 whose other end is located by the plug 56, bears on the needle valve 61. The trigger l2 exerts its action on a shoulder 68 of the needle valve 61. A rod 69 extending rearwardly from the rod 61 permits adjustment of the maximum opening by screwing or unscrewing of the plug 56 to vary the point of abutment. The shoulders of the air and paint valves where the action of the trigger 12 is exerted are however offset longitudinally so as firstly to obtain, upon opening, complete admission of air and then admission of paint. The action is vice versa for closing, firstly a cut-off in the supply of paint then that of the air.

The high voltage charges at the output from the generator are collected on an electrode 70 and from there are transported by a screw 71 leading to a conductor 72 made of a pliable material in contact with the member 59 also forming a charge electrode for the paint.

It will be noted that during operation of the gun whether it be when the trigger 12 is pulled or released, the excitation cable 40 is always retained under voltage. As soon as the air is put into circulation, the electro-aerodynamic generator supply voltage at the output by the simple fact of this circulation of the atomization.

It will also be noted that the cartridge 30 is easily interchangeable and that the guide finger 44 engaging in a slot locks this cartridge in a suitable angular position so as to make the passage 52 coincide with the annular space 51 for supplying compressed air. Instead of this arrangement an annular groove can be provided in the cartridge 30 at the location of the passage 51 so that the cartridge 30 can be engaged in any angular position.

in the modification shown in FIG. 6 a paint supply tube 79 is connected to the conduit 62 by providing the tube with an annular tip 80 which is pressed against a sealing member 81 by a screw collar 82.

lclaim:

1. In an electrostatic spray gun incorporating an electroaerodynarnic generator comprising:

means defining a convergent-divergent passage;

electrode means adjacent said convergent-divergent passage;

means for applying a voltage to said electrode means;

a conduit connected to said convergent-divergent passage;

and

means for feeding a gaseous fluid to said convergent-divergent passage so that said fluid picks up electric charges from said electrode means and flows through said con vergent-divergent passage and down said conduit at least partially at supersonic speed;

the improvement which comprises means for shutting off the flow of said gaseous fluid at a point downstream of said generator.

2. An electrostatic spray gun as claimed in claim 1, in which said gas-feeding means supplies said gaseous fluid under pressure.

3. Electrostatic spray gun as claimed in claim 1, comprising a hand grip, a trigger adjacent said hand grip, and a nozzle downstream of said generator, in which said means for shutting off said gaseous flow comprise a value positioned adjacent said nozzle and a member made of insulating material connecting said valve to said trigger.

4. Electrostatic spray gun as claimed in claim 3, in which said spray gun has a second passage for a fluid other than said gaseous fluid, and a second valve in said second passage, and said trigger is also connected to operate said second valve.

5. A spray gun as claimed in claim 2, in which said conduit has a downstream zone which is maintained at a high potential during operation of the gun.

6. In an electrostatic spray gun comprising generator means for accelerating at least part of a stream of gaseous fluid to supersonic speed and for imparting an electrical charge to said gaseous fluid, and means for supplying said gaseous fluid to said generator under pressure, the improvement which comprises means for shutting off the flow of gaseous fluid downstream of said generator, whereby the gaseous fluid in said generator is maintained under pressure despite said shutting off.

7. A spray gun as claimed in claim 6, comprising a nozzle through which at least part of the gaseous fluid emerging from said generator emerges from said gun, and means for supplying an additional fluid to said nozzle, said means for shutting off the flow of said gaseous fluid being positioned to control the flow of gaseous fluid from said generator to said nozzle.

l i l i 

1. In an electrostatic spray gun incorporating an electroaerodynamic generator comprising: means defining a convergent-divergent passage; electrode means adjacent said convergent-divergent passage; means for applying a voltage to said electrode means; a conduit connected to said convergent-divergent passage; and means for feeding a gaseous fluid to said convergent-divergent passage so that said fluid picks up electric charges from said electrode means and flows through said convergent-divergent passage and down said conduit at least partially at supersonic speed; the improvement which comprises means for shutting off the flow of said gaseous fluid at a point downstream of said generator.
 2. An electrostatic spray gun as claimed in claim 1, in which said gas-feeding means supplies said gaseous fluid under pressure.
 3. Electrostatic spray gun as claimed in claim 1, comprising a hand grip, a trigger adjacent said hand grip, and a nozzle downstream of said generator, in which said means for shutting off said gaseous flow comprise a value positioned adjacent said nozzle and a member made of insulating material connecting said valve to said trigger.
 4. Electrostatic spray gun as claimed in claim 3, in which said spray gun has a second passage for a fluid other than said gaseous fluid, and a second valve in said second passage, and said trigger is also connected to operate said second valve.
 5. A spray gun as claimed in claim 2, in which said conduit has a downstream zone which is maintained at a high potential during operation of the gun.
 6. In an electrostatic spray gun comprising generator means for accelerating at least part of a stream of gaseous fluid to supersonic speed and for imparting an electrical charge to said gaseous fluid, and means for supplying said gaseous fluid to said generator under pressure, the improvement which comprises means for shutting off the flow of gaseous fluid downstream of said generator, whereby the gaseous fluid in said generator is maintained under pressure despite said shutting off.
 7. A spray gun as claimed in claim 6, comprising a nozzle through which at least part of the gaseous fluid emerging from said generator emerges from said gun, and means for supplying an additional fluid to said nozzle, said means for shutting off the flow of said gaseous fluid being positioned to control the flow of gaseous fluid from said generator to said nozzle. 